How to add domain admin user of different domains to domain admin groups between two different trusted domains

I am trying to add a domain admin of domain 1 to a domain admins group of domain 2. I delegated control of Active directory of domain 1 and 2 to eachother's administrator users but when I try to add domain admin to admin admin groups of domain 2 I am unable to find the domain 2 listed under Locations.domain-admins.jpg

You cannot add users from another domain to Domain Admins because it is a domain local group and you cannot change it to a Universal group.
What you may be able to do is create a new universal security

What have been said is correct. You cannot add directly to the domain admins group in another domain due to be Domain Local Group. You can however like Joshua already state is add the users from the first domain into a group in the second domain. The difference here is I would not create a Universal Group because this will then be replicated forest wide using Global Catalog which will increase replication.

I personally would use a Global Group because it is more controlled as well because you can only add users and not groups. Having Universal is nice but can be dangerous if you added an entire group to a Domain Admin enabled security group.

Forest trust allow you to access resources and manage other domains. You cannot use clustering across different forests. Also I would NOT be installing an FTP server on your DC big security risk. Another thing is a DC has more restrictive security anyways which will also cause issues with your FTP server is setup to share specific directories.

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